HMS Queen Elizabeth, the Royal Navy’s new aircraft carrier, will take part in exercises in the South China Sea, Boris Johnson has pledged.
Photograph: Murdo Macleod for the Guardian

Boris Johnson has committed the UK’s two brand new aircraft carriers to freedom of navigation exercises in the fiercely contested waters of the South China Sea.

In a pointed declaration aimed squarely at China, whose island-building and militarisation in the sea has unnerved western powers, the British foreign secretary said that when the ships came into service they would be sent to the Asia-Pacific region as one of their first assignments.

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“One of the first things we will do with the two new colossal aircraft carriers that we have just built is send them on a freedom of navigation operation to this area,” Johnson said in Sydney on Thursday, “to vindicate our belief in the rules-based international system and in the freedom of navigation through those waterways which are absolutely vital for world trade.”

He later reiterated the point in a speech – only to confuse listeners by adding in a Q&A: “We haven’t yet quite decided to do that … but they are coming.”

The South China Sea is one of the busiest commercial sea routes in the world, carrying $5tn worth of trade a year. China claims it enjoys exclusive control over a massive portion of the sea – within the so-called nine-dash line – based on ancient rights marked in 600-year-old mariners’ books.

In dispute with four neighbouring countries, China has claimed sovereignty over islands in international waters in the sea, and built up sandbars and atolls into usable land. Several key islands have been populated and militarised with airfields, weapons systems and ports.

At 280 metres and 65,000 tonnes, the UK’s newest aircraft carrier, HMS Queen Elizabeth, is the largest ship ever built for the Royal Navy. It is undergoing its maiden sea trials off the coast of Scotland and is expected to be accepted by the navy towards the end of the year.

The second ship in the class, HMS Prince of Wales, is being fitted out in the Rosyth dock and will be officially named in September.

Speaking in Sydney after discussions with his Australian counterpart, Julie Bishop, Johnson said the UK and Australia had reaffirmed “our shared dedication to the rules-based international system that has been the guarantor of stability and, of course, also of prosperity for the last 70 years.”

He added: “In a volatile and unpredictable world it is more important than ever to nurture the friendships that we know best and that matter to us the most and with people we trust the most.”

Bishop said the ministers had discussed the South China Sea as one of the pressing “challenges” of the Asia-Pacific region. “We had a long discussion about the Pacific and the opportunities for deeper British engagement in our part of the world … we also see the United Kingdom as being a natural partner with us in the development and security of the Pacific,” she said.

In a speech in Sydney later in front of hundreds of politicians and businesspeople, Johnson reiterated the pledge to send the carriers.

“If you look at those vessels – you will see that they are not only longer than the entire Palace of Westminster but, I think you will agree, they are more persuasive than most of the arguments deployed in the House of Commons,” he said.

“And we pledge to do this [send the carriers] not because we have enemies in the region … but because we believe in upholding the rule of law.”

However, within minutes of the speech he appeared to sow some confusion over his promise. In a question-and-answer session he said: “We haven’t yet quite decided to do that. OK?” He went on: “I don’t want you to to go out and start scanning the horizons. But they are coming, they are coming. Don’t expect them tomorrow.”

The dialogue between the UK’s foreign and defence secretaries and Australia’s foreign and defence ministers – awkwardly appellated Aukmin – focused heavily on matters of security, intelligence and counter-terrorism.

Australia and the UK will strengthen arrangements to share classified information on defence, security and counter-terrorism operations and look to conduct cooperative military activities in the Asia-Pacific region.

The two countries will also discuss – based on the UK’s Operation Tempera and legislation proposed this month in Australia – the deployment of soldiers in domestic counter-terrorism operations.

The two countries will co-chair an international workshop on human trafficking and modern slavery, to be held at Australia House in London, in preparation for UN consultations on a global migration compact.

The ministerial dialogue also discussed trade, particularly the beginnings of a free trade agreement between Australia and the UK, to be struck after the UK engineers its exit from the European Union. Australia will sign a free trade agreement first with Europe, then with the UK.

Johnson said he expected, in negotiating a UK-Australia free trade deal, significant pressure to agree to a relaxing of visa conditions for Australians travelling to and working in the UK.

“I certainly support an open and generous regime in favour of our Australian friends being able to come, and our European friends, why not? Provided we control it,” he said.

Asked whether Australian wine might become cheaper in the UK as a result of a free trade deal, Johnson said it was a “cruel deprivation of the Australian people” that a tariff was imposed on Scotch whisky by Australia.

He said Australian wine was “delicious” and there were no tariffs on the importation of Bundaberg rum into the UK.

• This article was amended on 31 July 2017. An earlier version referred to the HMS Queen Elizabeth, and the HMS Prince of Wales. This has been corrected because Her Majesty’s Ship does not need the definite article.